This invention relates to fluorescent lamps of the type in which mercury is the source of primary radiation for phosphor excitation. In such a fluorescent lamp an electrical discharge is generated in mercury vapor at low pressure and typically mixed with argon gas. The light output from the lamp depends, among other variables, on the mercury vapor pressure inside the lamp tube. The primary radiation from the mercury is at 2537 Angstroms and arises from the transition between the lowest non-metastable excited state and the ground state. This ultraviolet radiation at 2537 Angstroms excites a phosphor which is coated inside the tube walls. The excited phosphor thereupon emits radiation at some wavelength, in the visible spectrum, characteristic of the phosphor.
It is known to the prior art that the optimum mercury pressure for maximum light output of a fluorescent lamp in alternating current operation is approximately 7 mtorr, which corresponds to a mercury cold spot temperature of approximately 40.degree. C. Exact values of the optimum vapor pressure and temperature are a function of the lamp tube radius. At cold spot temperatures higher or lower than the optimum light output falls off. The optimum temperature for AC operation is comparatively independent of the current applied to the lamp. That is to say, radiant intensity from an AC fluorescent lamp is at maximum at the approximate 40.degree. C optimum temperature apparently without regard to the applied current. "Cold spot" is used herein to mean that place where the fluorescent lamp tube is coolest and where the mercury is condensed.